Electronic musical instruments such as electronic organs presently generally employ divider circuitry known as "top octave frequency synthesizers" for generating twelve output signals which have the frequencies corresponding to the musical tones of the highest octave of the instrument. These output signals then are divided down by simple divider circuits to provide musical tones of the next highest octave and so on through all of the octaves for the lower octaves which are capable of production by the organ or other electronic musical instrument.
The frequencies of the tones produced in each of the octaves by such a system are accurate and are accurately octavely related to one another. A problem exists, however, in that the phase relationships of the tones in each of the octaves from a single top octave synthesizer are precisely locked in phase. The result of this is musically unpleasant and unnatural, and at times can result in the loss of certain notes or tones which the musician desires to have reproduced. For example, it is possible to play an 8' and a 4' flute, play octaves and hear only the lowest and highest note, the middle note being completely cancelled out by one keyer being on when the other is off. This would be the situation when the harmonics of the notes are subtractive. On the other hand, the harmonics can be additive which would unnaturally emphasize the middle note.
A solution to the problem of phase-locked tones in top octave synthesizer systems has been to provide a number of top octave synthesizers, equal to each of the maximum number of tones which are likely to be produced by the musical instrument at any one time, and to have independent oscillators providing the source of clock pulses or drive signals for each different top octave synthesizer in the instrument. In such a system, twelve high-frequency stable oscillators, such as crystal oscillators, are required for each keyboard if the maximum possibility or probability of notes from the instrument at any one time is equal to twelve. Even so, care must be taken that two or more of these oscillators do not become phase-locked to one another; and this is difficult and expensive to implement.
It is desirable to provide a top octave synthesizer system for an electronic organ or electronic musical instrument which produces an output chorus tone effect comparable to the effect obtained from sounding corresponding tones by means of different musical instruments. When tones produced by different musical instruments are combined in an orchestra, the various tones are not precise harmonics of one another but differ slightly, which produces a pleasing natural chorus tonal effect to the listener.
Accordingly, it is desirable to produce a tonal effect in an electronic organ employing top octave synthesizers which simulate this more natural imprecise harmonic effect to the listener and which utilizes a minimum number of precision oscillators.